Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
Thereby, care has to be taken that a specimen under investigation is not damaged. Thus, low voltage microscopy is important for the imaging of sensitive and non-conductive specimen. Due to the low energy (typically lower than 5 keV) of the primary charged particles, resulting in low energy dissipation, sensitive specimens are not damaged. Additionally, the charging behavior of the insulating specimen can be advantageous for low voltages because the secondary electron emission can be controlled to be equal to the primary electron absorption of the specimen. Low voltage microscopy is, thus, interesting for the dimensional measurement and the inspection of device structures in the semiconductor manufacturing process.
Presently, high resolution low voltage microscopes are used for the above mentioned applications. Prior art systems, as e.g. described in EP-B-0 333 018, use a combined electrostatic-magnetic immersion lens as final objective lens. Immersion lenses allow for high beam energies within the column and lower beam energies on impingement on a specimen. Thereby, reduction of the Boersch effect and reduced beam landing energies can be combined.
The back-scattered and/or secondary charged particles released on impingement of primary charged particles on a specimen can be detected by a detector located within the objective lens or above the objective lens. This arrangement of an in-lens or pre-lens detector has the advantage that the specimen can be located very close to the lens, resulting in a short working distance. A short working distance results in improved imaging properties, especially improved resolution.
State of the art systems, as e.g. described in U.S. Pat. No. 5,780,859, have a drawback in secondary electron detection efficiency, since an immersion lens accelerates the secondary charged particles to a potential comparable to the deceleration potential for the primary charged particle beam. In view of the high secondary charged particle energy, the detection of these particles is more difficult. Therefore, state of the art solutions either use coaxial detectors with small holes for the penetration of the primary beam (EP-B-0 333 018) or means for separation of the primary and the secondary electron beam (U.S. Pat. No. 5,422,486).
It is an object of the invention to provide a charged particle beam device overcoming the problems and disadvantages in the state of the art. Furthermore, it is an object of the present invention to provide a method for operating a charged particle beam device that overcomes at least some of the problems in the state of the art.